In Unity, import the two plugins: NGUI (tested with 2.3.6) and Toolkit2D (tested with 1.91final)

Optionally, if you want to build with Ouya, you need to get the latest Ouya SDK for Unity (currently 1.0.3)

Once both plugins are imported, from the menu, look for M8, then click on 'Meta GUID Restore'

Look for Assets/guids.txt in the Project view and drag it to the box next to 'Select'

You will notice that two buttons will appear: 'Generate' and 'Restore'

Click on 'Restore', cross your fingers

Once that's done, Unity will re-import assets. All should be well at this point

Check any prefabs or scenes to see if there are missing references, hopefully there's none...Otherwise it may be that certain script files no longer exist if you imported from a higher version of NGUI and/or Toolkit 2D. You'll have to resolve those manually.

(NOTE: This is sort of a work-around in Unity since any references to scripts, materials, etc. are based on the generated UIDs. There are no true reliable way to reconnect these missing references when the GUIDs are lost. The danger is the fact that this will restore from previously generated GUIDs which means there is a chance that any of those GUIDs will conflict with newer GUIDs generated by the project. I'm not sure if Unity takes care of this under the hood whenever new GUIDs are generated. So keep that in mind!)